Existing studies of middle childhood suggest that some children with ASD show significant improvements in IQ and other aspects of cognitive functioning during this period, while others stabilize or fall further behind. With the increased availability of intensive early intervention, it is not clear that findings from these older studie are applicable to children with ASD now. Given that cognitive abilities developed through early and middle childhood provide a critical foundation for later competencies, and are the strongest predictors of adolescent and adult outcomes, the lack of clarity about cognitive development during middle childhood constitutes a significant gap in our understanding of ASD. To close this gap we propose to conduct a longitudinal study of individuals with ASD that focuses on middle childhood in a well-characterized cohort of children from the UC Davis MIND Institute Autism Phenome Project (APP). One hundred and eighty nine toddlers with ASD and 90 with typical development (TYP) were assessed during early childhood (at 2-3.5 years or T1). A subset of these children received additional behavioral assessments when they were 5-6.5 years old (T2) and provide pilot data for the application. In the proposed study, we will reassess the original cohort of 279 when they reach middle childhood (at 8-10 years or T3). In Aim #1 we will a) examine cognitive functioning; and b) identify distinct developmental trajectories of intellectual functioning/IQ between early and middle childhood and predictors of membership in these trajectories. We hypothesize that they will show 3 trajectories of intellectual development based on our pilot data and the literature. These include the: 1) the Stable Low developmental trajectory; 2) the Stable High developmental trajectory; and 3) the Changers developmental trajectory. We hypothesize that membership in the Stable High group at T3, will be predicted by better executive attentional control and fewer autism-related social affect symptoms at T1; and that membership in the Changers group at T3, vs. the Stable Low group, will be predicted by the absence of regression and fewer autism-related communication symptoms at T1. In Aim #2, we test 2 mechanistic models of how intensive early intervention might promote positive middle childhood functioning in individuals with ASD. These include the Hippocampal Compensation Model, which proposes that in some children intervention promotes neuroplasticity of the hippocampus which then leads to improvements in memory, IQ, and academic functioning in middle childhood and the Social Attention Model, which suggests that in children with better academic, social, and adaptive functioning at T3 there will be a positive association between intensity of early intervention, and executive attentional control and social communication development. This application addresses IACC Strategic Plan Questions 2 and 4; and the NIMH RDoC recommendation to study executive attentional control.